Skip to main content

Full text of "Heterothallism in Bread Mold, Rhizopus nigricans"

See other formats


Early Journal Content on JSTOR, Free to Anyone in the World 

This article is one of nearly 500,000 scholarly works digitized and made freely available to everyone in 
the world by JSTOR. 

Known as the Early Journal Content, this set of works include research articles, news, letters, and other 
writings published in more than 200 of the oldest leading academic journals. The works date from the 
mid-seventeenth to the early twentieth centuries. 

We encourage people to read and share the Early Journal Content openly and to tell others that this 
resource exists. People may post this content online or redistribute in any way for non-commercial 

Read more about Early Journal Content at 
journal-content . 

JSTOR is a digital library of academic journals, books, and primary source objects. JSTOR helps people 
discover, use, and build upon a wide range of content through a powerful research and teaching 
platform, and preserves this content for future generations. JSTOR is part of ITHAKA, a not-for-profit 
organization that also includes Ithaka S+R and Portico. For more information about JSTOR, please 



In recent publications the writer has shown that, according to their 
method of sexual reproduction, the Mucorineae may be divided into two 
groups — homothallic and heterothallic. In a homothallic species, zygo- 
spores are formed from branches of the same mycelium and can be obtained 
from the sowing of a single spore. In heterothallic species, which probably 
constitute the large majority of the Mucorineae, zygospores are formed 
from branches which necessarily come from two different mycelia belonging 
to sexually opposite races or strains. By sowing a single spore, therefore, 
one cannot obtain zygospores of a member of the heterothallic group unless 
the culture becomes contaminated with spores of the opposite strain. 

Within the last few months, two communications have appeared which 
claim that the production of the zygospores of Rhizopus is not at all depend- 
ent upon the presence of two sexual races; in other words, that on the 
proper nutrient the zygospores can be obtained from the sowing of a single 
spore. This laboratory weed, the common bread mold, is the best known 
species among the mucors and has been used as a type of the heterothallic 

A propos of the article by Hamaker (Science N. S. 23:710. 1906), the 
writer has recently summarized the results of a series of investigations on 
the sexual condition in this species covering a period of over six years 
(Science N. S. 24:118-122. 1906). Since this was written, the prelim- 
inary communication of Hamaker has been reviewed in the Botanical 
Gazette (42:77. 1906) where the detailed formula is repeated for making 
the corn muffin bread claimed to be a sure medium for the production of 
the zygospores of this species. It is not the purpose of the writer to repeat 
what has been said already in regard to the inadequacy of this nutrient 
theory of zygospore formation. It seems desirable, however, to consider 
somewhat in detail the paper of Namyslowski {Rhizopus nigricans et les 
conditions de la formation de ses zygospores: Bull. Acad. Sci. Cracovie 
676-692. pi. 21. 1906), since this investigator explicitly casts doubt upon 
the occurrence of heterothallism in any of the Mucorineae and would give 
the appearance of supporting his conclusions by cultural experiments with 
415] [Botanical Gazette, vol. 43 


It may be said at the start that, as every laboratory mycologist knows, 
Rhizopus shares with Penicillium the distinction of being the worst fungus 
weed in laboratory cultures, and is practically certain to come as a con- 
tamination of substrata rich in carbohydrates unless they be sterilized and 
kept under sterile conditions. In the Harvard laboratory, as in most other 
botanical institutions, Rhizopus is obtained for class use by spontaneous 
infection of bread. Moreover, the sporangium wall is brittle and when 
ruptured the spores are discharged into the air. It is somewhat difficult, 
therefore, to obtain a pure transfer from a single sporangium without con- 
tamination with spores of other sporangia near by, and, before the discovery 
of heterothallism in this form, the writer more than once obtained zygo- 
spores from transfers which at the time were thought to have come from 
single sporangia. 

Namyslowski's conclusions are based for the most part on two series 
of cultures. In the first, 19 bread cultures were made from spores from 
one single sporangium, and zygospores were produced in every case. In 
the second series, 46 single spores were isolated by the separation method 
and used in the inoculation of cultures chiefly of bread. Of these 6 were 
destroyed by bacteria, 13 produced only a feeble growth of mycelium 
(probably also infected by bacteria), 13 only sporangia, and 14 zygospores. 
The failure to obtain zygospores in so large a number of the sowings is 
ascribed to improper moisture content of the air in the cultures and to other 
unfavorable conditions which are not further investigated. 

Two explanations are possible for these results: (1) that Namy- 
slowski was in fact dealing with a homothallic form of Rhizopus and zygo- 
spores were the result of single spore sowings; (2) that infection from the 
air brought the ( + ) and (— ) strains into contact and caused the production 
of zygospores. Mr. H. A. Edson, of the Vermont Experiment Station, has 
recently sent the writer zygospores of Rhizopus which he writes were 
secured synthetically by opposing two strains, one obtained from Vermont, 
the other from Virginia. A dozen strains were tested by him from as many 
different sources, but failed to produce zygospores alone in pure cultures. 
The writer himself has tested 60 different strains from localities in such 
widely separated parts of the world as North and South America, England, 
Germany, France, Honolulu, and the Philippine Islands, and has found 
none which alone in pure cultures will produce zygospores. A homothallic 
form of the ordinary species, therefore, would seem a priori rather improb- 
able, and to be proven only by careful cultures strictly under sterile 

Namyslowski says nothing about sterilizing his culture media, and the 


fact that out of 46 cultures on bread, 6 (over 13 per cent.) were destroyed by 
bacteria and 13 more were probably also infected by bacteria, since they 
produced only a scanty mycelium devoid of fructifications, shows that 
Namyslowski's cultures were far from pure, and renders it probable that, 
in the 14 instances in which zygospores were obtained out of the 46 cul- 
tures, the ( + ) and ( — ) strains had become present through infection; 
while in the 13 cultures in which only sporangia were obtained, whatever 
infection occurred was of the same strain as the spores used in the inocula- 
tion. The 19 cultures on bread from the same sporangium would give 
zygospores if the inoculating material became infected by spores of the 
opposite race before sowing, even though subsequent infection did not occur. 

That zygospores may be obtained by accidental infection of a favorable 
substratum with both (+) and ( — ) spores has been repeatedly demon- 
strated by the writer's own experience in the Harvard laboratory. Pro- 
fessor Campbell, moreover, has written that for ten years he has gotten 
zygospores by spontaneous infection of suitable substrata, and an investi- 
gation showed that in zygosporic material sent from his laboratory only the 
two (+) and ( — ) strains were present which alone were incapable of 
zygospore formation. 

Since the above was written, Dr. Namyslowski has kindly sent the 
writer zygosporic material of the Rhizopus nigricans which he used in his 
experiments. Inoculations were at once made into sterilized nutrient and 
a large number of the young zygospores resulting were isolated and laid on 
cleared nutrient agar in Petri dishes. Paired cultures X & XX and Y & 
YY were obtained by making mycelial transfers from the outgrowths from 
the suspensors of two zygospores both of whose suspensors had germinated. 
On the assumption that the species is heterothallic, each pair should contain 
the two opposite strains, and each strain when grown alone should be incap- 
able of zygospore formation. In addition, cultures Z & ZZ were obtained 
from suspensors that could not be traced with certainty to the same zygo- 
spore, and cultures A-D were obtained from the germination of single 
isolated sporangiospores. By opposing inoculations from these cultures 
against one another and against standard ( + ) and ( — ) test strains, it was 
seen that X and Y are (-) and XX, YY, Z, ZZ, and A-D are (+). The 
contrasts numbered over 40 and were made for the most part on sterilized 
flour paste in stender dishes, but were controlled by cultures on nutrient 
agar in test tubes. None of the strains produced zygospores on substrata 
favorable to zygospore formation when sown alone in pure cultures, nor 
when sown together with the same strain from a different source. They 
have always formed them, however, whenever the opposite strains are grown 




The form is therefore hetero- 

together under proper nutrient conditions 

Whether or not it may ever be found that a homothallic race may occur 
in a species normally heterothallic (perhaps not an impossible condition in 
view of the writer's having obtained a homothallic mycelium of the hetero- 
thallic Phycomyces), the evidence at hand leads one to the conclusion that 
the large majority of the Mucorineae are heterothallic. — A. F. Blakeslee, 
Cryptogamic Laboratory, Harvard University. 


(with one figure) 

The presence of more than one archesporial cell in the megasporangium 
of the Spermatophyta has frequently been reported for gymnosperms and 
dicotyledons, but the records of their occurrence in monocotyledons are 

very few. In 1882 Guignard 1 
figured two small hypodermal 
cells in Ornithogalum pyrena- 
icum, which he interprets as 
archesporial. As these cells 
are but little larger than the 
other cells of the nucellus, and 
the character of the proto- 
plasmic content of the adjacent 
cells is not shown, the evidence 
here is not altogether conclu- 
sive. Coulter and Cham- 
berlain 2 state that they have 
observed two preparations of 
Lilium philadelphicum show- 
ing respectively three and five 
archesporial cells, but they 
neither described nor figured 
Bernards observed and figured two four-celled embryo sacs 

1 Guignard, L., Recherches sur le sac embryonnaire des Phanerogames Angio- 
spermes. Ann. Sci. Nat. Bot. VI. 13:136-199. pis. 3-7. 1882. 

2 Coulter and Chamberlain, Morphology of Angiosperms. New York. 1903. 

3 Bernard, C. H., Recherches sur les spheres attractives chez Lilium caniiium, 
etc. Jour. Botanique 14:178. pis. 4-5. 1900. 

Fig. 1. — Vertical section through a young 
ovule of Lilium longiflorum, showing two 
embryo-sac mother cells.